1. Field of the Invention
The invention relates to magnetic article surveillance systems, and coded markers for such systems, which are capable of generating and distinguishing among large numbers of codes.
2. Statement of Art
Article surveillance systems using soft magnetic materials and low frequency detection systems have been known since the Picard patent (Ser. No. 763,861) was issued in France in 1934. Picard discovered that when a piece of metal is subjected to a sinusoidally varying magnetic field, an induced voltage, characteristic of the metal composition, is produced in a pair of balanced coils in the vicinity of the applied field. Today, such systems utilize the harmonics produced by a marker of soft magnetic strip to detect the marker. Due to the nonlinear characteristics of such markers, groups of even and odd order harmonics can be produced simultaneously or individually. Odd order (1, 3, 5 . . . ) harmonics are produced by a symmetrical switching of the B/H loop. Even order harmonics (2, 4, 6 . . . ) are produced by a non-symmetrical switching condition, typically caused by a D.C. magnetic bias internal or external to the material.
The nonlinear characteristics of the soft magnetic material, while not commonly found, can be duplicated is some ferrous alloys by the presence of a magnetic bias. This results in the generation of even and odd order harmonics that duplicates the response of soft magnetic materials, such as permalloy and the metallic glass products. However, the use of more sensitive detection equipment can add to the probability of false alarms due to ferrous alloys.
Another limitation of the soft strip and low frequency system is that only a single bit of information is available during marker and system interaction. The marker is either in the detection zone, or not. The only other alternative is that the marker is, whether or not in the detection zone, deactivated. While this is not a disadvantage for systems used in theft control, it is an extreme limitation when used for monitoring the flow of a group of differing objects, or even persons, through the detection zone.
Those systems using coded devices for monitoring people and articles in a selected area are quite capable of a large number of codes. Card access systems are a good example. They generally combine a digital network and/or radio frequency circuit to transmit the code. However, these devices are too expensive to use either for theft control of low cost items or for inventory control in factories or stores. It is understood that encoded markers can be affixed to or otherwise carried by any article or person, animal, etc. The term "article" is used herein to encompass such possibilities.
This invention differs from the prior art in that the codes utilized are not duplicated by biased ferrous alloys, even accidentally. Further, the coded marker can be embodied in a single element device and can be programmed (code changed) by altering the geometry of or extent of a conductor surrounding a magnetic core. It is detectable at large distances and is not sensitive to spatial orientation within the system. The number of codes does not depend on the marker structure but on the phase resolution of the detection system and programming device.
The concept of this invention can best be appreciated in contrast to the teachings of specific and representative patents. The prior art can be broken down into the following classifications: (1) single element; (2) multiple element (3) biased (magnetized); and, (4) unbiased.
A bistable magnetic device is disclosed in U.S. Pat. No. 3,820,090-Wiegand. The marker is in the form of a wire, preferably with a magnetically "hard" magnetized outer shell (having a relatively high coercivity) and a moderately "soft" magnetic core (having a relatively low coercivity). The magnetized shell portion is operable for magnetizing the core portion in a first direction, the magnetization of the core portion is reversible by application of a separate magnetic field and the shell is operable to remagnetize the core portion in the first direction upon removal of the separate magnetic field. The device requires a fixed orientation to the interrogation field. The system can produce additional codes only by using multiple elements. Such devices are generally used for close proximity card access systems.
The device disclosed in U.S. Pat. No. 3,747,086-Peterson uses multiple elements to bias a soft magnetic strip. The marker comprises a plurality of ferromagnetic elements including a first element capable of generating a signal containing harmonics of an exciting oscillatory interrogating field and a seond element having a coercive force greater than the first element and capable of retaining a state of magnetization when exposed to the interrogation field, such that when so magnetized, a magnetic bias is imposed on the first element to prevent the generation of the harmonic signal. Four possibilities (codes) exist depending on which element is magnetized. However, these codes are easily reproduced in any biased, ferrous alloy. The system is neither unique nor reliable.
The system disclosed in U.S. Pat. No. 3,765,007-Elder uses markers of "n" number of elements with differing AC coercivities to produce "n" number of codes. When the elements are subjected to a periodically varying magnetic field, the magnetization of the elements reverses sequentially at equal intervals of time. Like Peterson, Elder's system is prone to false alarms from biased, ferrous alloys which inadvertently, and all to frequently, duplicate the code. Moreover, a plurality of magnetic field producing means must be used to cover all orientations of the coded elements (markers).
The system disclosed in U.S. Pat. No. 4,134,538-Lagard, et al. uses markers of "n" multiple elements or bands producing varying amplitudes as a code. Such magnetic bands are selectively divided at variable predetermined locations by cuts of variable predetermined extent, such that when in the detection zone, signals of varying amplitudes are produced. The marker must pass correctly oriented and in close proximity to the coils in the detection zone. It is primarily a device intended for access or inventory control and is expensive to produce.
This invention is based upon the discovery that when a suitable conductor, such as aluminum or copper, partially or totally encloses a core of soft magnetic material, the phase of the harmonics produced will be shifted (delayed in time). The amount of phase shift induced is controlled largely by the amount and resistivity of the conductor surrounding the magnetic material. It is feasible to shift any harmonic or groups of harmonics by any amount, through 360 degrees. However, some loss of harmonic amplitude is encountered as the conductor thickness increases and as the harmonic number increases.
The ability to control harmonic phase permits the generation of signals having a unique signature, apart from both ferrous alloys and soft magnetic materials. This avoids the accidental detections plaguing prior art systems as described above. In addition, a number of codes can be established according to the phase shift induced. The phase shift is not affected by a low level, external magnetic bias, in that odd order products are totally unaffected and even products shift by +/-180 degrees.
The system comprises an oscillator which provides phase locked signals to a transmitter/amplifier circuit and receiver/phase comparator circuit. Phase shifted harmonic generated by the marker are captured and amplified in the receiver. A comparison is made only of the phase of the received harmonics to the phase of the transmitted signal. Either one or more harmonics may be compared depending on the particular system use. A system used for theft control device requires a minimum code level but a maximum number of harmonic phase comparisons. A system used for inventory control would require a maximum number of codes but a minimum number of phase comparisons. Once the phase shift is compared and found to be correct (in the case of a theft system for example) an alarm is sounded. An inventory control system would have further processing equipment to send data to a cash register or a computer, to actuate a mechanical/electrical device or a combination thereof.